Rasmussen also said Russia's military buildup on the Ukrainian border remains a "grave concern."
He was speaking after a NATO foreign ministers meeting in Brussels today.
The Russian Foreign Ministry says the two top diplomats, in a telephone conversation today, also discussed possibilities for international cooperation in the crisis over Ukraine but gave no details.
Yesterday, NATO foreign ministers said they were suspending "all practical civilian and military cooperation" with Russia in protest at its annexation of Ukraine's Crimea.
NATO also said it will intensify cooperation and help improve Ukraine's military capabilities through training and other programs.
The alliance said it would also look at ways to enhance collective defense, including through the possible deployment or reinforcement of military assets in eastern NATO members such as Poland and the Baltic states.
A Russian Foreign Ministry spokesman Aleksandr Lukashevich said the NATO statements resemble the "verbal jousting of the Cold War era. "
Wondering what was in all those documents found at Viktor Yanukovych's palace? Buzzfeed, natch, has a list for that: http://t.co/1YNTrwfelS
— Mark MacKinnon (@markmackinnon) April 2, 2014
Talking to the on-line Dozhd (Rain) TV station today, Yanukovych said he cannot agree with Crimea’s separation from Ukraine and added that he would have done all he could to prevent the peninsula's annexation by Moscow if he had been still in power.
Yanukovych said he hopes to persuade Russian President Vladimir Putin to discuss the conditions under which Crimea could rejoin Ukraine.
He added that presidential elections in Ukraine scheduled for May 25 will be illegal, as the "politically unstable state brings a big number of problems both into interior political situation in Ukraine and beyond."
Yanukovych reiterated his previous stance, saying that a referendum on Ukraine's federalization should be held before the presidential poll.
Associated Press: #Yanukovych says #Crimea a part of #Ukraine http://t.co/g0Ez0imb4t pic.twitter.com/vr0s7yuf3W
— KyivPost (@KyivPost) April 2, 2014
OK Yanukovych can you get back in your box now please, this is unbelievably boring.
— Shaun Walker (@shaunwalker7) April 2, 2014
yes, please, God RT @shaunwalker7: OK Yanukovych can you get back in your box now please, this is unbelievably boring.
— Maxim Eristavi (@MaximEristavi) April 2, 2014
Yanukovych now droning on about minute agriculture and heavy industry statistics. Positively soporific
— max seddon (@maxseddon) April 2, 2014
First copy on the @AP Yabukovych interview http://t.co/Vp2UnNoOt8
— Daniel Sandford (@BBCDanielS) April 2, 2014
MT @HromadskeTV: #Ukraine government planning to auction Yanukovych's fleet of luxury cars in June. (Ukr) pic.twitter.com/P9B5VctyzE
— Chrystyna Lapychak (@chryslap) April 2, 2014